The present invention relates to a novel process for the preparation of insecticidally active esters of the general formula(I): ##STR3## wherein R.sub.1 and R.sub.2 are the same or different from each other and represent a hydrogen or a halogen atom; R.sub.3 represents ##STR4## (wherein R.sub.4 and R.sub.5 represent a chlorine or bromine atom or a methyl group when R.sub.4 is identical to R.sub.5, but R.sub.4 represents a chlorine or bromine atom or a methyl group and R.sub.5 represents a trifluoromethyl group when R.sub.4 is different from R.sub.5, and R.sub.6 represents a halogen atom or a difluoromethoxy group); and R.sub.8 represents a hydrogen or a cyano group.
It is known, according to the following methods, that the publicly known process for the preparation of the compound of formula(I) wherein R.sub.8 is a cyano group, which is used for pesticides and acaricides.
(A) U.K. Patent Specification No. 1,540,632 discloses a process for the preparation of an ester of the general formula (3) wherein A is an optionally-substituted alkyl or cycloalkyl group and B is a phenoxy, phenylthio or benzyl, which comprises reacting a benzaldehyde of the formula (2) with an acyl halide of the formula (1) (wherein X.sub.1 is a bromine or chlorine atom) in the presence of water, a water-soluble cyanide, a substantially water-immiscible aprotic solvent and a phase transfer catalyst.
The reaction formula of the above process is as follows; ##STR5##
The acyl halide of formula (1) should be prepared by reacting a halogenation reagent such as thionyl chloride with an organic acid. However, because the halogenation reagent is sensitive to moisture, it is unstable in an atmosphere with a high moisture content and the reactor should be dried before each reaction. Also corrosion of the reactor occurs easily since hydrogen chloride and sulfurous acid gases are produced as byproducts. Thus, when they are not appropriately treated, they cause a pollution problem. Because the reactivity of the acyl halide of formula (1), an intermediate, is great and it decomposes readily in the presence of water and heat, purification is difficult and a specific apparatus must be used.
(B) German Patent No. 2,651,341 discloses a process for the preparation of an ester of the general formula (3) wherein A and B are the same as the above-mentioned, which comprises neutralizing an acid of the formula (4) with a water-soluble base and reacting the product with a halide of the formula (5) (wherein X.sub.2 is a halogen atom) in the presence of water, a substantially water-immiscible aprotic solvent and a phase transfer catalyst.
The reaction formula of the above process is as follows; ##STR6##
The halide of the formula (5) is a compound derived from an alcohol precursor whose industrial preparation is difficult.
Because the halogenation reagent must be used as mentioned in (A) so as that the alcohol is converted to the halide, there are the same problems as in (A).
(C) U.S. Pat. No. 4,409,150 discloses a method for preparing a pyrethroid insecticide of the general formula (9) wherein A.sub.1 is a cycloalkyl and B.sub.1 and B.sub.2 represent hydrogen or halogen atoms, in which an acid of formula (7) is neutralized with a water-soluble base and then reacted in the presence of a phase transfer catalyst with a solution in a substantially water-immiscible organic solvent of an alpha-cyanobenzyl arylsulphonate of formula (8) wherein B.sub.3 represents an optionally substituted aryl group. ##STR7##
The alpha-cyanobenzyl arylsulphonate of formula (8) is prepared by reacting a benzaldehyde and a water-soluble cyanide. After separating the sulphonates of formula (8), unstable intermediates, they are reacted with an acid of formula (7) in order to prepare an ester of formula (9) and then the complexity of the procedure itself becomes a difficult problem.
Also, pursuant to the following methods, the process for the production of the compound of formula (I) wherein R.sub.8 is a hydrogen is known.
(D) German Patent No. 2,326,077 discloses a process for the preparation of an ester of the general formula (12) comprising condensing an acyl halide of formula (10) with an alcohol of formula (11) in the presence of an organic base as represented in the following reaction formula. ##STR8##
The acyl halide of formula (10) must be prepared by reacting a halogenation reagent such as thionyl chloride with an organic acid. But as mentioned in the explanation of (A), there are some problems and drawbacks.
(E) German Patent No. 2,437,882 discloses a process for the preparation of an ester of the general formula (12), which comprises reacting a metal salt of an organic acid of formula (13) wherein M is an alkali metal, with a quaternary ammonium, as shown in the following reaction formula. ##STR9##
(F) German Patent No. 2,544,150 discloses a process for the preparation of an ester of the general formula (12) comprising reacting a lower alkyl ester of formula (15) with an alcohol of formula (11) in the presence of a titanium catalyst to trans-esterify, as represented in the following reaction formula. ##STR10##
In the above-mentioned prior arts (E) and (F), because the organic acid or one of the alcohol reactants is prepared by condensation after it has been made into a functional derivative and then activated, there are some inconvenient steps in the procedure.